1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. In this specification, the semiconductor device refers to any device which can function with the use of semiconductor characteristics.
2. Description of the Related Art
In recent years, there has been extensive production of semiconductor devices in which a thin film transistor (a TFT) is formed over a substrate such as a glass substrate which has an insulating surface and used as a switching element or the like. In the thin film transistor, an island-shaped semiconductor layer is formed over a substrate having an insulating surface by a CVD method, a photolithography process, or the like, and part of the island-shaped semiconductor layer is used as a channel formation region of the transistor (for example, see Reference 1: Japanese Published Patent Application No. H08-018055).
A schematic cross-sectional view of a thin film transistor is shown in FIG. 21. As shown in FIG. 21, in the thin film transistor, an insulating layer 31 which serves as a base film is formed over a substrate 30; a semiconductor layer 32 which includes a channel formation region 32a and impurity regions 32b and 32c each of which serves as a source region or a drain region is formed over the insulating layer 31; an insulating layer 33 which serves as a gate insulating layer is formed over the semiconductor layer 32 and the insulating layer 31; a conductive layer 34 which serves as a gate electrode is formed over the insulating layer 33; an insulating layer 203 is formed over the conductive layer 34; and wirings 204 electrically connected to the impurity regions 32b and 32c are formed over the insulating layer 203.